1. Field of the Invention
The present invention relates to an electrode for interventional purposes, such as a pacemaker or ICD electrode or an electrophysiological catheter, as is used, for example, with pacemakers, defibrillators, neurostimulators, and in procedures of electrophysiology (EP).
2. Description of the Related Art
Electrodes of this type are known to have an elongate electrode body having a distal end and a proximal end. At least one active electrode pole for delivering an intervention pulse is provided in the area of the distal end. This electrode pole is designed, for example, as a tip electrode pole situated directly at the distal end, as a ring pole placed at a distance therefrom, or as a shock electrode. The intervention pulses delivered via this pole are, for example, the pacemaker pulses of a cardiac pacemaker and/or neurostimulator, a high-voltage pulse in the case of a defibrillator, or an ablation energy pulse in the case of an ablation device.
A supply line comprising a first material, which produces a connection to this electrode pole, runs in the electrode body. Furthermore, such electrodes for interventional purposes usually have further electrode poles to thus be referred to, using which the electrode may come into contact with tissue. A ring electrode pole of a bipolar electrode or an EP catheter are cited as examples. Furthermore, an electrode sheath which encloses the at least one supply line is provided in the electrode body, which comprises a second material. This material is usually insulating and/or different from the first material.
In recent years, magnetic resonance (MR) diagnostic devices, also referred to in the following as MR devices (such as magnetic resonance tomographs=MRT) have gained great significance because of their examination methods, which are protective of patients, noninvasive, and completely free of pain and side effects. Typical electrodes for interventional purposes display the problem that electrodes of this type heat up strongly in magnetic resonance diagnostic devices under the influence of the electromagnetic radiation thus generated because of electromagnetic induction and the discharge of the induced energy in the area of their contact surface(s) to the tissue. The reason for this is particularly the solid, metallic supply lines to the electrode poles, which act as an antenna and in which, because of their insulation, the antenna currents induced by high frequency (HF) fields may only be discharged into the body electrolytes at the electrode poles, which form the electrical interface to the tissue. The cited HF fields operate, for example, in an operating frequency range of 21 MHz in a 0.5 Tesla MR tomograph. The operating frequency range may extend up to 300 MHz in 7 Tesla MR tomographs according to the current prior art and is typically at 64 MHz in a 1.5 Tesla MR tomograph, for example. Because extremely strong heating in the tissue may occur in proximity to the electrode pole, access to magnetic resonance diagnostic devices is typically blocked to wearers of cardiological and neurological intervention devices, such as cardiac pacemakers, neurostimulators, or defibrillators.
To prevent and/or minimize the hazardous heating of the body cells, the maximum antenna current must be limited and/or reduced. Known solutions suggest discrete components for this purpose, which act as a band-stop filter or as a low-pass filter and thus limit the conduction resistance of the antenna for the frequencies of interest. Other solutions suggest capacitors, which are connected in parallel to the insulation and thus dissipate the antenna current.
In this regard, for example, U.S. Pat. No. 6,944,489, US 2003/0144720 A1, US 2003/0144721 A1, US 2005/0288751 A1 (and the simultaneously published parallel publications US 2005/0288752 A1, US 2005/0288754 A1, and US 2005/0288756 A1, which have essentially the same wording) are cited.
In principle, this possibility exists, of influencing the inductance and capacitive coupling of the antenna by construction measures and thus reducing the flow of the antenna current, dissipating it, or shifting the resonance frequency. The construction requirements placed on the electrode from a therapeutic aspect only allow little play for this purpose, however.
Furthermore, in contrast to the very simplified consideration used here, antennas also have further resonance frequencies, so that the shift of the resonance behavior of their electrode may possibly again meet the resonance condition in an MR device using other HF frequencies. This path is therefore not advantageous.
Furthermore, EP 0 884 024 B1 is to be cited as technological background, in which a capacitor is connected between the supply lines for the ablation pole of an ablation catheter and a measuring pole, also situated thereon, for recording ECG signals. Because of this capacitor, high-frequency energy may be delivered for ablation via the ablation electrode and ECG signals may also be recorded simultaneously.
US 2006/0009819 A1 discloses a cardiac pacemaker having an elongate electrode which is connected to a pulse generator connector. A passive lossy circuit is provided, which is electrically connected between a distal section of the electrode supply line and a high-frequency grounded surface. The passive lossy circuit has a high-frequency impedance which is approximately equal to a characteristic impedance of the electrode in its implanted state in the body. The reflection of incident waves is thus minimized at the terminals of the lossy circuit and their energy is intentionally dissipated here. The passive lossy circuit also acts as a low-pass filter, because of which the electrode is functional in normal operation of the cardiac pacemaker.
A further example of such an electrode is described in US 2006/0200218 A1. A solution is suggested therein, in which the electrode sheath at least sectionally comprises frequency-dependent insulating material having a lesser conductivity at lower frequencies, while the at least one supply line is produced in a known way and comprises typical material. The frequency dependence of the electrode sheath is implemented by a polymer composite which has dielectric properties. A capacitive coupling of the electrode supply line to the external medium is thus possible. However, this capacitive coupling leaves high effectiveness to be desired. This is because the resistive frequency-independent component of such a capacitive coupling is constant, because of which the total impedance upon HF incidence of an electromagnetic wave may not assume arbitrarily small values.